Engines may use boosting devices, such as turbochargers, to increase engine power density. However, engine knock may occur due to increased combustion temperatures. Knock is especially problematic under boosted conditions due to high charge temperatures. The inventors herein have recognized that utilizing an engine system with a split exhaust system, where a first exhaust manifold routes exhaust gas recirculation (EGR) to an intake of the engine, upstream of a compressor of the turbocharger, and where a second exhaust manifold routes exhaust to a turbine of the turbocharger in an exhaust of the engine, may decrease knock and increase engine efficiency. In such an engine system, each cylinder may include two intake valves and two exhaust valves, where a first set of cylinder exhaust valves (e.g., scavenge exhaust valves) exclusively coupled to the first exhaust manifold may be operated at a different timing than a second set of cylinder exhaust valves (e.g., blowdown exhaust valves) exclusively coupled to the second exhaust manifold, thereby isolating a scavenging portion and blowdown portion of exhaust gases. The timing of the first set of cylinder exhaust valves may also be coordinated with a timing of cylinder intake valves to create a positive valve overlap period where fresh intake air (or a mixture of fresh intake air and EGR), referred to as blowthrough, may flow through the cylinders and back to the intake, upstream of the compressor, via an EGR passage coupled to the first exhaust manifold. Blowthrough air may remove residual exhaust gases from within the cylinders (referred to as scavenging). The inventors herein have recognized that by flowing a first portion of the exhaust gas (e.g., higher pressure exhaust) through the turbine and a higher pressure exhaust passage and flowing a second portion of the exhaust gas (e.g., lower pressure exhaust) and blowthrough air to the compressor inlet, combustion temperatures can be reduced while improving the turbine's work efficiency and engine torque.
However, the inventors herein have recognized potential issues with such systems. As one example, during the positive valve overlap period where both intake valves and a scavenge exhaust valve (e.g., one of the first set of cylinder exhaust valves) are open, some blowthrough air may “short circuit” directly from an intake valve to the scavenge valve without thoroughly scavenging residual burned exhaust gases from a combustion chamber of the cylinder. The inventors have recognized that though staggering the timing of the two intake valves may reduce this short circuiting, this may require additional hardware, thereby increasing engine costs and a complexity of engine control.
In one example, the issues described above may be addressed by a method, comprising: in response to flowing gases from engine cylinders to an intake passage via a first set of exhaust valves, adjusting a first set of swirl valves coupled upstream of a first set of intake valves to at least partially block intake air flow to the first set of intake valves, where each cylinder includes two intake valves including one of the first set of intake valves and two exhaust valves. As one example, each cylinder may include one of the first set of swirl valves disposed in an intake port of one of the intake valves. Under certain conditions, including when engine load is greater than a threshold load and/or when a valve disposed in an exhaust gas recirculation (EGR) passage coupled between the intake passage and a first exhaust manifold coupled to the first set of exhaust valves is open, a controller of the engine may adjust the first set of swirl valves to at least partially block intake air flow to the first set of intake valves. As a result, this may increase turbulence of intake air flow entering the cylinders via the first set of intake valves, thereby increasing the scavenging of the residual burned exhaust gases from the combustion chambers. In this way, engine emissions may be reduced and engine efficiency may be increased.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.